The invention relates generally to wireless communications systems. More particularly, the invention relates to providing high-speed broadband services to a large number of users by utilizing a minimum amount of bandwidth in a wireless communications system.
The need for high-speed broadband packet services will grow tremendously in the coming years as work-at-home, telecommuting and Internet access become increasingly popular. Customers are expecting high quality, reliability and easy access to high-speed communications from homes and small businesses. Data rates of at least 10 mega-bits per second (Mbps) are needed to provide high speed services for: a) accessing the World Wide Web (WWW) for information and entertainment, b) providing data rates comparable to local-area networks (LAN) for telecommuters to access their computer equipment and data at the office, and c) multimedia services such as voice, image and video.
Traditional wireless communications systems have a problem delivering high-speed services because of the amount of bandwidth these services require. Bandwidth is a key limiting factor in determining the amount of information a system can transmit to a user at any one time. In terms of wireless networks, bandwidth refers to the difference between the two limiting frequencies of a band expressed in Hertz (Hz).
The concept of bandwidth may be better understood using an analogy. If information carried by a network were water, and links between communication sites were pipes, the amount of water (i.e., information) a network could transmit from one site to another site would be limited by the speed of the water and the diameter of the pipes carrying the water. Ignoring speed for a moment, the larger the diameter of the pipe, the more water (i.e., information) can be transmitted from one site to another in a given time interval. Likewise, the more bandwidth a communications system has available to it, the more information it can carry.
Traditional wired communications systems using modems and a physical transmission medium such as twisted pair copper wire, cannot currently achieve the data rates necessary to deliver high-speed service due to bandwidth limitations (i.e., small pipes). In an attempt to solve this bandwidth problem, the local exchange companies (LEC) have been engaged in planning and deploying hybrid fiber/co-ax (HFC) and switched digital video (SDV) networks. These wired-network approaches to providing high-speed access, however, require a substantial market penetration to keep the per-subscriber costs at an acceptable level due to the high costs involved.
Similarly, traditional wireless systems such as narrowband cellular and Personal Communications Services (PCS) are bandwidth limited as well. As an alternative, wireless solutions such as Multichannel Multipoint Distribution Service (MMDS) and Local Multichannel Distribution Service (LMDS) have become attractive for low take-rate scenarios, e.g., a market penetration of a few percent. The benefits of wireless systems for delivering high-speed services is that they can be deployed rapidly without installation of local wired distribution networks. The problem with MMDS and LMDS, however, is that these solutions presently offer limited uplink channel capacity Moreover, these solutions may not be capable of supporting a large number of users due to limited frequency reuse.
One solution for solving the bandwidth limitation problem for wireless systems is to maximize the available bandwidth through frequency reuse. Frequency reuse refers to reusing a common frequency band in different cells within the system. The concept of frequency reuse will be discussed in more detail with reference to FIGS. 1 and 2.
FIG. 1 is a diagram of a typical wireless communication system. A typical wireless communications system includes a plurality of communications sites, such as mobile telephone switching office (MTSO), base stations, terminal stations, or any other site equipped with a radio transmitter and/or receiver.
FIG. 1 shows a base station 20 in communication with terminal stations 22. Base station 20 is usually connected to a fixed network, such as the PSTN or the Internet. Base station 20 could also be connected to other base stations, or connected to a MTSO in the case of mobile systems. Terminal stations 22 can be either fixed or mobile.
Base station 20 communicates information to terminal stations 22 using radio signals transmitted over a range of carrier frequencies. Frequencies represent a finite natural resource, and are in extremely high demand. Moreover, frequencies are heavily regulated by both Federal and State governments. Consequently, each cellular system has access to a very limited number of frequencies. Accordingly, wireless systems attempt to reuse frequencies in as many cells within the system as possible.
To accomplish this, a cellular system uses a frequency reuse pattern. A major factor in designing a frequency reuse pattern is the attempt to maximize system capacity while maintaining an acceptable signal-to-interference ratio (SIR). SIR refers to the ratio of the level of the received desired signal to the level of the received undesired signal. Co-channel interference is interference due to the common use of the same frequency band by two different cells.
To determine frequency reuse, a cellular system takes the total frequency spectrum allotted to the system and divides it into K frequency reuse patterns. FIGS. 2(A) through 2(D) illustrates examples of frequency reuse patterns of K=4, 7, 12 and 19.
As shown in FIGS. 2(A) through 2(D), a cellular communications system has a number of communications sites located throughout a geographic coverage area serviced by the system. This geographic area is organized into cells and/or sectors, with each cell typically containing a plurality of communications sites such as a base station and terminal stations. A cell is represented in FIGS. 2(A) through 2(D) as a hexagon. FIG. 2(A) shows a frequency reuse pattern where K=4. Cells are grouped into sets of four, with each set employing frequency bands 1 through 4. This group of four cells is then repeated until the entire service area is covered. This same pattern is shown in FIGS. 2(B), 2(C) and 2(D) for sets of 7, 12 and 19 cells, respectively. Thus, in essence, the frequency reuse pattern represents how much geographic distance must be maintained between cells that use common frequency bands such that the co-channel interference for these cells is kept below a given threshold to ensure successful signal reception.
The most aggressive frequency reuse pattern for cellular systems is where K=1. Under this pattern, the same frequency band can be reused in every cell in the cellular communications system. In typical narrowband cellular systems, the total amount of frequency spectrum available to a system is divided by K. This determines how much frequency is available for a particular cell. For example, if a cellular system is allocated 20 megahertz (MHZ) of spectrum, and the frequency reuse pattern is K=4, then each cell has 5 MHZ worth of frequency on which to transmit radio signals. If K=1, the entire 20 MHZ worth of frequency spectrum is available to every cell to potentially transmit information.
To better understand the magnitude of benefit given by a frequency reuse pattern of K=1 discussed in the above example, the figures for a real communications system will be used. The frequency assignment for U.S. mobile cellular systems is 824-849 MHZ and 869-894 MHZ for a given service area. Since each service area is served by two cellular network operators, each cellular system must split the available bandwidth for the given service area. This amounts to a total of 25 MHZ of available bandwidth per system, with 12.5 MHZ being used for transmitting from a base station to a terminal station (referred to as the downlink), and 12.5 MHZ being used for transmitting from the terminal station to the base station (referred to as the uplink). A typical U.S. mobile cellular system has a frequency reuse pattern of K=21. Thus, each cell has roughly only 1.2 MHZ (25 MHZ divided by 21) of spectrum to transmit information. If a frequency reuse pattern of K=1 could be established, the entire 25 MHZ is available for transmitting information for each cell. This results in a twenty-one fold increase in available frequency spectrum for each cell. Using the analogy again, the diameter of the pipe is increased twenty-one times.
Several existing systems currently employ frequency reuse patterns of K=1. One example includes cellular systems employing code division multiple access (CDMA). CDMA systems spread the transmitted signal across a wide frequency band using a code. The same code is used to recover the transmitted signal by the CDMA receiver. CDMA systems reuse the same frequencies from cell to cell. CDMA systems, however, require a large amount of frequency spectrum. Moreover, the amount of spectrum required by CDMA systems to offer high-speed broadband services to a large number of users is commercially unrealistic.
Another example for aggressive frequency reuse includes cellular systems employing time division multiple access (TDMA), an example of which is discussed in U.S. Pat. No. 5,355,367. The system discussed in U.S. Pat. No. 5,355,367 is a TDMA system using the redundant transmission of information packets to ensure an adequate SIR for a call. The use of redundant packet transmissions, however, merely trades one inefficiency for another. Although a frequency band can be reused from cell to cell, redundant packet transmission means that a smaller portion of that frequency band is now available for use by each cell in the system since multiple packets are required to ensure the successful reception of a single packet.
In addition to the frequency reuse problem, traditional cellular systems are not engineered to allow a communications site to use the entire bandwidth available to the system ("total system bandwidth"), due to the low data rate expected by customers. Rather, traditional cellular systems employ various techniques in both the frequency domain and time domain to maximize the amount of users capable of being serviced by the system. These techniques are predicated on allocating smaller portions of the total system bandwidth to service individual communication sites. These smaller portions are incapable of providing sufficient bandwidth to offer high speed services.
An example of a technique employed in the frequency domain is Frequency Division Multiple Access (FDMA). FDMA splits the available bandwidth into smaller sections of bandwidth under the concept of providing less bandwidth for a greater number of users. Using the analogy, a single large pipe is separated into a number of smaller pipes, each of which is assigned to a sector or cell Unfortunately, the trade-off is that these smaller frequency bands are not large enough to support high-speed broadband packet services. Moreover, by definition, a communication site is not capable of using the total system bandwidth, but rather is limited to a discrete portion of the total system bandwidth.
An example of a technique employed in the time domain is TDMA. TDMA divides the available bandwidth into discrete sections of time, and allocates each section of time (typically referred to as a time slot) to each communication site. Each communication site transmits and receives information only at the site's specific time slot, thereby preventing collisions between communication sites. Using the analogy, each cell or sector has access to the entire pipe for a fixed amount of time. Traditional TDMA systems, however, are designed to handle circuit switching and, therefore, are static in nature. These systems allocate a specific time slot of a fixed duration for a specific communication site for the entire length of a call. As a result, a communication site cannot transmit more information than can be accommodated by its assigned time slot. In any event, these traditional TDMA systems are not designed to take advantage of new switching technology, such as packet switching.
Some systems employ a combination of FDMA and TDMA to improve the call capacity of the system. These FDMA/TDMA systems, however, merely combine the disadvantages of both. Moreover, FDMA/TDMA systems do not permit a user access to the total system bandwidth on a dynamic basis.
To solve this problem, some TDMA systems employ a concept called "dynamic resource allocation" to share the radio resource among communications sites efficiently. Dynamic resource allocation methods, however, require a central controller or complicated algorithms to dynamically determine available time slots and coordinate their use.
In light of the foregoing, it can be appreciated that a substantial need exists for a system employing a frequency reuse pattern of K=1 while allowing a communication site to utilize the total system bandwidth on a dynamic basis, thereby providing high-speed broadband packet services to a large number of users while minimizing the amount of required bandwidth.